1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser and particularly to a surface emitting semiconductor laser and a manufacturing method thereof, by which laser oscillation with low active current and high efficiency can be realized by decreasing reactive current and effectively utilizing the emitted light.
2. Description of the Prior Art
Semiconductor lasers have been put into practical use in optical communication, laser printers, photo-disk players etc. A semiconductor laser manufactured and practically utilized at present has an active region in a specified surface of a semiconductor wafer, parallel to the major surface of the semiconductor wafer and utilizes, as mirrors, a pair of plane surfaces which are obtained by cleavage and intersect orthogonally with the above stated major surface. In consequence, as is different from an ordinary semiconductor device, such a semiconductor laser involves problems in that characteristic tests and passivation can be applied only after separation into a tip state by cleavage. Because of such problems, it is difficult at present to mass-produce semiconductor lasers having the ordinary structure.
In order to solve such problems, inventive or experimental technique have been considered for forming mirrors by etching, instead of cleavage and surface emitting semiconductors to which the present invention is related. Such surface emitting semiconductor lasers are disclosed for example by H. Soda, K. Iga, C. Kitahara and Y. Saematsu in "GaInAsP/InP Surface Emitting Injection Lasers", Japan Journal of Applied Physics., Vol. 18, No. 12, pp. 2329-2330 (1979). However, it is difficult to form, by etching, mirrors as plane as those formed by cleavage and a method for realizing a surface emitting semiconductor laser capable of obtaining laser oscillation by low active current is not yet developed. Under these circumstances, little progress is made for drastically improving semiconductor lasers using cleavage. For example, a surface emitting semiconductor laser utilizing cleavage, in which an active layer in the form of a convex lens is provided in the interior is disclosed by K.Shima et al. in "Buried Convex Waveguide Structure (GaAl)As Injection Lasers", Appl. Phys. Lett., Vol. 38(8), pp. 605-606 (15 Apr., 1981). However, this semiconductor laser cannot realize surface light emission.
A sectional view of a conventional surface emitting semiconductor laser examined by experiment is shown in FIG. 1. The semiconductor laser in FIG. 1 comprises an n-type semiconductor substrate 10 having a surface 11 formed as a mirror and another surface 12 plane and parallel to the surface 11. An n-type first semiconductor layer 20 is epitaxially grown on the surface 12 and a p-type active layer 30 is epitaxially grown on this first semiconductor layer 20. The interface between the first semiconductor layer 20 and the active layer 30 forms a PN junction J.sub.1. A p-type second semiconductor layer 40 is epitaxially grown on the active layer 30 and one surface 41 thereof functions as a mirror. A negative electrode 50 is formed on the above described surface 11 of the semiconductor substrate 10 so that it may be in the form of a ring having an opening 51 in the center. On the surface 41 of the above described second semiconductor layer 40, there is formed an insulating film 60, which has an extremely small contact hole 61 in a position opposed to the central portion of the above described opening 51 of the negative electrode 50. A positive electrode 70 is formed on the insulating film 60 and is electrically connected to the surface 41 of the second semiconductor layer 40 through the contact hole 61 of the insulating film 60. The active layer 30 is formed of a material having a forbidden band width smaller than the respective forbidden band widths of the materials forming the semiconductor substrate 10, the first semiconductor layer 20 and the second semiconductor layer 40.
Now, description will be made of the operation of a surface emitting semiconductor laser thus structured. First, when voltage is applied between the positive electrode 70 and the negative electrode 50, current i.sub.1 flows between the positive electrode 70 and the negative electrode 50. This current i.sub.1 has a high current density in the vicinity of the positive electrode 70 and a low current density in the vicinity of the negative electrode 50, respectively, as shown by the dotted lines in FIG. 1. On the other hand, carrier confinement effect is generated in the active layer 30 since the active layer 30 is interposed between the p-type second semiconductor layer 40 and the n-type first semiconductor layer 20 and is formed of a material having a forbidden band width smaller than the respective forbidden widths of the semiconductor substrate 10, the first semiconductor layer 20 and the second semiconductor 40. As a result, concentration of the holes and electrons injected into the active layer 30 increases, and if an amount of current i.sub.1 exceeding a threshold value flows, stimulated light emission is caused. Consequently, light moves back and forth repeatedly between a pair of mirrors formed by the surface 41 of the second semiconductor layer 40 and one surface 11 of the semiconductor substrate 10. Such optical path is shown by the chain line in FIG. 1, where light l.sub.1 moves back and forth in the direction shown by the arrows and is amplified each time it passes through the active layer 30. Repetition of the amplification causes laser oscillation and a part of the laser light generated as the result is emitted from the surface 11 of the semiconductor substrate 10 to the exterior, as shown by the arrow L.sub.1.
However, since the surface emitting semiconductor laser thus structured does not have a function of converging light in the direction of an optical axis perpendicular to the active layer 30, most of the generated light is dissipated and only an extremely small amount of light is utilized for stimulated light emission. Accordingly, active current and a heat generation in this surface emitting semiconductor laser become extremely large. In consequence, pulse oscillation can be barely performed at a low temperature of approximately 77 K., and at the room temperature, the threshold current is 1.5 A and the electric power efficiency is 0.1%, resulting in high active current and low efficiency.
In addition, if laser oscillation is caused, the carrier concentration in the region subjected to the oscillation is decreased due to the recombination of electrons and holes and light moves to other regions of high carrier concentration, since light amplification and laser oscillation are caused by the recombination of electrons and holes in the portions where the concentration of electrons and holes is high. More specifically, the optical path always moves and is never fixed, which makes the oscillation mode unstable.